This invention concerns a modified high silica-to-alumina ratio acidic crystalline zeolite catalyst of a special group such as ZSM-5 having increased activity and decreased aging rate when employed in a variety of hydrocarbon conversion processes.
It is well known in the petroleum refining art to improve the quality of various hydrocarbon oils by treating them with catalysts under varying conversion conditions to effect such reactions as cracking, hydrocracking, hydrofining, isomerization, dewaxing, and the like. In these processes, operating catalyst life usually depends on the nature of feedstock, the severity of the operation and often, on the nature and the extent of operational upsets. Gradual catalyst deactivation is countered by incrementally raising the operating temperature to maintain the required conversion.
Numerous investigators have demonstrated that the activity of silica-alumina and clays used in various acid-type catalytic reactions depends in part on the degree of hydration of the surface. The effect of water on hydrocracking was reported by T. Y. Yan (see Journal of Catalysis 25, 204-211 (1972)). Yan evaluated the addition of water, introduced as 2-pentanol or water vapor, on the hydrocracking activities of palladium impregnated rare earth exchanged zeolite X and a platinum impregnated zeolite HY. The addition of 3 wt % 2-pentanol to the feed or saturating the feed with water vapor at 75% reduced the temperature required for a 60% conversion of n-hexadecane by 12.degree. F. Yan further showed that the activation was due to water and not by the pentene produced by the dehydration of 2-pentanol. The promotional effect of water on the hydroprocessing of a commercial feedstock was found to be minimal. Furthermore, water failed to promote zeolite HY. Minachev, et al. have reported in Soviet Scientific Reviews, Section B., Chemistry Reviews, Vol. 2, 1-6 (1980) that sodium forms of zeolites are not promoted by water. In addition, Ward (Journal of Catalysis, 11,238-250 (1968) studied the influence of small amounts of water on the acidity of several alkali, alkaline earth, hydrogen and mixed cation zeolites by observing changes in the infrared spectrum of chemisorbed pyridine. Water had no marked effect on the acidity of alkali cation X and Y zeolites.
Water and water precursors have also been disclosed in the patent art as useful in enhancing catalytically promoted petroleum processes. Water has been disclosed as enhancing the activity of metal catalysts supported on inorganic metal oxide supports in such processes as reforming (U.S. Pat. Nos. 2,642,383 to Berger, et al. and 3,649,524 to Derr, et al.); hydrodesulfurization (U.S. Pat. No. 3,720,602 to Riley, et al.); dehydrogenation (U.S. Pat. No. 3,907,921 to Winter) and hydrocracking (U.S. Pat. No. 4,097,364 to Egan). In addition, water has been found useful in petroleum processing in promoting the activity of crystalline aluminosilicates, such as zeolite X and Y (U.S. Pat. Nos. 3,943,490 to Plank, et al. (cracking); 3,546,100 to Yan (hydrocracking) and 4,097,364 to Egan (hydrocracking)) and ZSM-5 (U.S. Pat. No. 4,149,960 to Garwood, et al. (dewaxing)). Garwood, et al. disclose that dewaxing of gas oils with a ZSM-5 type zeolite in hydrogen form is enhanced by cofeeding water with the gas oil feed. The benefits obtained are improvements in coke laydown and catalyst aging rates. There is no suggestion that a sodium exchanged ZSM-5 type zeolite has catalytic dewaxing capability or that the presence of water would benefit such a zeolite.
In some particular petroleum conversion processes involving cracking a certain class of compounds in a feedstock may be converted to modify a characteristic of the whole feedstock. Exemplary of the latter type of conversion is catalytic hydrodewaxing whose principle purpose is to reduce the pour point of wax containing mineral oils. Pour point is the temperature at which an oil will not flow, as determined by standardized test procedures. The wax compounds are long carbon chain molecules which tend to crystallize on cooling of the oil to an extent such that it will not flow, hence it may not be pumped or transported by the pipelines at ambient temperatures.
Catalytic dewaxing as practiced today involves the shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms, viz., the waxy molecule, to reduce the pour point, pumpability and/or viscosity of mineral oil fractions which contain these waxy constituents.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Pat. No. Re. 28,398 of Chen, et al. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the waxes. However, only minor conversion occurs in dewaxing. For example, Chen et al. describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9% conversion.
Among the less specialized techniques for producing products of lower molecular weight than the hydrocarbon charge stock are catalytic cracking and catalytic hydrocracking. Catalytic cracking involves contacting the heavy hydrocarbon charge with a porous acidic solid catalyst at elevated temperatures in the range of 850.degree. to 1000.degree. F. to yield the desired lower boiling liquid product of greater value than the liquid charge (e.g. motor gasoline) together with normally gaseous hydrocarbons and coke as byproducts. Hydrocracking employs a porous acidic catalyst similar to that used in the catalytic cracking but associated with a hydrogenation component such as metals of Groups VI and VIII of the Periodic Table. An excess of hydrogen is supplied to the hydrocracking reactor under superatmospheric pressure at lower temperatures than those characteristic of catalytic cracking, say about 650.degree. F.
Since the introduction of zeolite catalysts as exemplified by U.S. Pat. No. 3,140,249, a large proportion of the capacity for catalytic cracking and hydrocracking has been converted to use of such highly active catalysts. The high activity zeolite catalysts are characterized by very low content of alkali metal. Sodium, for example, is present as a cation in synthetic faujasites by reason of their manufacture. Expensive ion exchange operations are carried out in the preparation of cracking and hydrocracking catalysts from synthetic faujasite to replace the sodium or other alkali metal by protons or poly-valent metal cations, especially rare earth metal cations.
It has been recognized that such zeolites can function as catalysts when containing a moderate percentage of sodium. Thus, U.S. Pat. No. Re. 26,188 to Kimberlin, et al. exhibits data showing cracking activity of a faujasite from which only one-third of the sodium has been removed by ion exchange. The extremely high activity of such catalysts as zeolite ZSM-5 has been moderated for specialized purposes by using the zeolite in the partially sodium form. See, for example, U.S. Pat. No. 3,899,544.
Zeolite ZSM-5 preparation is described in U.S. Pat. No. 3,702,886 which also describes several processes in which the zeolite is an effective catalyst, including cracking and hydrocracking. That zeolite is shown to be prepared from a forming solution which contains organic cations, namely alkyl substituted ammonium cations. Those large organic cations then occupy cationic sites of the zeolite and block pores at least partially. The conventional method for removing the organic cations is to burn them out with air at elevated temperature, leaving a proton at the site previously occupied by the organic cation. Sodium, or other alkali metal, at other cationic sites may then be ion exchanged to provide protons or multivalent metals as desired to prepare catalysts for cracking, hydrocracking and other purposes.
The acid activity of zeolite catalysts is conveniently defined by the alpha scale described in an article published in Journal of Catalysis, Vol. VI, pp 278-287 (1966). In this test, the zeolite catalyst is contacted with hexane under conditions prescribed in the publication and the amount of hexane which is cracked is measured. From this measurement is computed an "alpha" value which characterizes the catalyst for its cracking activity for hexane. The entire article above referred to is incorporated herein by references. The alpha scale so described will be used herein to define activity levels for cracking n-hexane. And, in particular, for purposes of this invention, a catalyst with an alpha value of less than about 10 and preferably less than about 1 will be considered to have substantially little activity for cracking n-hexane.
The shape selective catalysis of zeolites is defined by the Constraint Index scale described in an article published in the Journal of Catalysis, Vol. 67, pp 218-222 (1981). In this test, the zeolite catalyst is contacted with a mixture of hexane and 3-methylpentane under conditions set forth in the publication and the amount of hexane and 3-methylpentane cracked is measured. From this measurement a constraint index value is computed which is related to the ability of the zeolite for shape selective catalysis. The entire article above is incorporated herein by references. The contraint index scale so described will be used herein to describe the ability of zeolite for shape selective catalysis.
U.S. Pat. No. 4,247,388 to Banta, et al. discloses that the catalytic performance of certain acidic zeolites such as those of the ZSM-5 type in hydrodewaxing operations is improved by controlling the alpha activity of such zeolites to within the range of 55-150, e.g., by treatment with steam.
U.S. Pat. No. 4,284,529 to Shihabi discloses improvements in pour point reduction by means of catalytic dewaxing employing a catalyst prepared from a ZSM-5 type zeolite having a constraint index of about 1 to 12. This dewaxing process employs a low acidity form of zeolite such as ZSM-5 or ZSM-11 in which the low acidity is imparted by steaming the zeolite to reduce its cracking activity to an alpha value of not less than about 5, followed by base ion exchange with an alkali metal cation to reduce the alpha value to not greater than 1.0. A preferred catalyst is referred to therein as a presteamed Na ZSM-5 and is employed to dewax crude oils and other waxy feedstocks in the presence or absence of added hydrogen. These catalysts are effective at start-of-run temperature of about 640.degree. F. and exhibit excellent aging behavior in the presence of hydrogen. However, in the absence of hydrogen these catalysts exhibit a gradual aging requiring a daily increase of about 1.degree.-10.degree. F. in the reaction temperature. Dewaxing processes conducted with presteamed sodium ZSM-5 in the absence of hydrogen exhibit, on the average, cycle times of several weeks between catalyst regenerations because of catalyst aging.
The presteamed base exchanged catalyst disclosed in U.S. Pat. No. 4,284,529 is particularly suited to reducing the pour point of waxy crude oils. This catalyst is especially resistant to the metals, nitrogen and sulfur often associated with crude oils and it does not cause the formation of appreciable quantities of C.sub.3 gaseous products so that the liquid recovery from crude dewaxing is often 98% or better. Ideally, crude oil dewaxing should be practiced at well-head so as to permit easy transporting of the dewaxed crude by pipeline. Where an economical source of hydrogen is available, the above described process is commercially feasible. However, practicing this process without a source of hydrogen could be economically attractive if the cycle times between catalyst regenerations are sufficiently long.